1. Field of the Invention
This invention relates to the field of fluxes in particular direct chip attach (DCA) flux material. Of particular interest within the present invention are fluxes of the type suitable for use in processes where the article to be fluxed is dipped in a reservoir of the flux material prior to placement and soldering. These are commonplace for “package on package” (PoP) electronic assemblies and the like.
2. Description of Related Technology
Soldering processes are well-known. These range from hand-soldering methods to machine soldering methods. It is well-known to use a flux material with the soldering material. The flux material satisfies a number of functions. Generally a main function of the flux material is to ensure that conductive material of a first component, for example a support substrate such as a printed circuit board (PCB) interconnects well with good electrical conduction across the solder interconnect to a second component such as an electronic component. Utilising solder alone will not form an interconnect.
To form a successful interconnection it is necessary to apply a fluxing material. A number of different alternatives have been used. It is known for example to include a solid flux material within the soldering material. Generally such a soldering material would then be provided in a wire or other such solid form which would incorporate a core of flux material running through the solder. As the solder melts upon heating, the flux is activated, and the resulting interconnect formed if the soldering process is of an acceptable standard.
It is also known to use solder paste materials. Solder pastes are generally homogenous, stable suspensions of solder particles in a flux medium material.
Reflow soldering is used in the automated manufacture of PCB's, wherein electronic components are surface mounted on PCB's to which a solder paste material has previously been applied by a method such as screen or stencil printing or dispensing. The PCB is then subjected to a sufficiently high temperature, to cause the solder to liquefy and to join the components in place on the PCB. The heat can be supplied by, for example, infrared, heated conveyor belt or convective means.
In light of circuit miniaturization and complexity increasingly demanded by modern electronics devices, for example mobile telephones, soldering processes have emerged such as those that can be applied by a pick-and-dip process. For example a flux in the form of a tacky fluid is employed. A tacky fluid in this context is one that can be applied by a pick-and-dip process but which is sufficiently tacky to remain in place on a dipped component for subsequent soldering.
The pick-and-dip process is typically used for components that already have pre-applied solder. Many electronic components are manufactured with sufficient solder on-board the component to allow the component to be subsequently soldered to a substrate. The substrate may be a circuit board or indeed another electronic component.
It is desirable to apply the tacky flux directly to a component that has been manufactured with solder already in place. Tacky flux may, for example, be applied to a ball grid array (BGA) or a bumped application specific integrated circuit (ASIC).
Typically this application of the tacky flux is achieved by dipping in a reservoir. The reservoir will typically be very shallow and dipping will immerse only the pre-applied solder to the desired depth.
Once the tacky flux is applied, the component can be placed on the substrate such a PCB prior to reflow. Generally the tacky flux provides green strength (holding the component in place before soldering). Such tacky fluxes are generally substantially colourless or may have a slight colouring imparted by the components of the composition. For example if rosin is used it may impart slight yellow to orange colouring when a relatively large volume of composition is viewed. In the small quantities applied to the solder the flux is substantially colourless and is generally not seen.
In the past the majority of solder compositions used were of the tin-lead type which proved useful in most common applications. The flux materials of the present invention may be employed with such solder compositions.
The use of lead in solder composition has come under scrutiny and has become undesirable for environmental and health reasons. As a result, attempts have been made to eliminate the requirement for lead from solder compositions. Such lead-free solder compositions typically include tin, copper and silver (typically 95.5% by weight tin, 4% by weight silver and 0.5% by weight copper). Bismuth may also be used together with tin, antimony and silver in a range of approximately 1.0% to 4.5% by weight. Patents directed to lead-free alloys include U.S. Pat. Nos. 1,437,641, 3,607,253, 4,042,725, 4,170,472, 4,667,871, 4,670,217, 4,695,428, 4,758,407, 4,778,733, 4,806,309, 4,879,096, 4,929,423, 5,094,813, 5,102,748, 5,147,471, 5,242,658, 5,256,370, 5,316,205, 5,320,272, 5,328,660, 5,344,607, 5,352,407, 5,390,080, 5,393,489, 5,405,577, 5,411,703, 5,414,303, 5,429,689, 5,435,968, 5,439,639, 5,452,842, 5,455,004, 5,527,628, 5,538,686, 5,569,433, 5,580,520, 5,658,528, 5,698,160, 5,718,868, 5,730,932, 5,733,501, 5,755,896, 5,762,866, 5,817,194, 5,837,191, 5,843,371, 5,851,482, 5,863,493, 5,874,043, 5,918,795, and 6,231,691; European Patent Documents EP-A-0 251 611, EP-A-0 336 575, EP-A-0 629 463, EP-A-0 629 464, EP-A-0 629 465, EP-A-0 629 466, and EP-A-0 629 467; Great Britain Patent Document GB-A-2,158,459; Japanese Patent Documents JP-A-5050286, and JP-A-8230598; and International Patent Publication WO-A-94/2563 the disclosures of each of which are hereby expressly incorporated herein by reference.
There has been a specific requirement to achieve reliable attachment of components in the pick-and-place processes. In this respect the correct application of flux is a critical consideration. If sufficient flux is not applied (for example insufficient amounts being used or insufficient coverage with flux) effective soldering may not be achieved at each solder point. This ultimately can lead to faulty interconnects/circuits being created which is undesirable.
It has been generally recognised in the art that providing a flux material which achieves a number of functions is desirable. In particular it is desirable that the flux material provides good surface activation. In this respect, it has been known to include an activator component within the flux material which will act to remove oxidised material from a metal surface, thereby allowing better solder to metal interconnection and ultimately metal (PCB) to metal (electronic component) interconnection.
Activators are added to decompose and remove any oxide film existing in the portion where soldering is going to be carried out, and are usually organic compounds containing halides, typically amine hydrohalides, such as diethylamine hydrochloride or weak halide-free organic acids.
In particular, it is desirable to use a carboxylic acid which melts at a temperature in the range of from 75 to 250 degrees Celsius as described in European Patent Publication No. 0 620 077 A.
It would be of particular importance to more reliably apply flux to components particularly those of the type having pre-applied solder carried thereon. This will in turn mean lower rejection rates of the assembled electronic device.